I always thought that smaller horses had the advantage in reining. Like cow horses (after all they are pretty much the same), reining horses can get low to the ground a heck of a lot easier if they are 14-14.3hh-ish. I can see how being smaller would make it easier to do roll backs, sliding stops, and even the spins.
But can bigger horses still play the game? What would your cut off size be when looking at a horse's potential in reining?